One thing that has worried me with multistrand wire is whether there can be
arcing or intermod if one conductor in the bundle has a break in continuity.
This could also arise if the centre wire in a 7 strand bundle is not
soldered properly. It is very hard to tell if solder has penetrated "right
through", especially for wire that has been out in the open and weathered
somewhat. What I am saying is that at RF there can be potential differences
due to transmission line effects, and a conductor that has one end floating
is like a stub, but as the gap is small it can easily arc over. Arcing
could cause broadband QRM.
So while soldered joints in antenna wires appear to be fair game, it would
pay to have the individual wires cleaned to a "bright copper" condition
before firing up the solder bolt.
I have seen crimps used to join antenna wires, and crimping could be suspect
for multistrand wire.
I would not worry about losses in soldered joints, but as Markus has
commented, corrosion could be a limitation in the longer term. In any case,
you will probably want to experiment with changes to your antenna well
before corrosion has a chance to show up :-)
73, Bob ZL2CA